Posttranslational modification of eukaryotic proteins, particularly therapeutic proteins such as immunoglobulins, is often necessary for proper protein folding and function. Because standard prokaryotic expression systems lack the proper machinery necessary for such modifications, alternative expression systems have to be used in production of these therapeutic proteins. Even where eukaryotic proteins do not have posttranslational modifications, prokaryotic expression systems often lack necessary chaperone proteins required for proper folding. Yeast and fungi are attractive options for expressing proteins as they can be easily grown at a large scale in simple media, which allows low production costs, and yeast and fungi have posttranslational machinery and chaperones that perform similar functions as found in mammalian cells. Moreover, tools are available to manipulate the relatively simple genetic makeup of yeast and fungal cells as well as more complex eukaryotic cells such as mammalian or insect cells (De Pourcq et al., Appl Microbiol Biotechnol, 87(5):1617-31).
However, posttranslational modifications occurring in yeast and fungi may still be a concern for the production of recombinant therapeutic protein. In particular, O-mannosylation is one of the biggest hurdles to overcome in the production of biopharmaceuticals for human applications in fungi. More specifically, yeasts like Pichia pastoris and Saccharomyces cerevisiae tend to hyper-mannosylate heterologously expressed biopharmaceuticals, thereby triggering adverse effects when applied to humans.
O-mannosylation to Serine and Threonine residues includes in mammals GalNAc based oligosaccharides or GlcNAc/N-acetyllactosamine comprising O-linked mannose glycans. In fungi O-mannosylation occurs as hexose monomers or oligomers. In yeasts, there are typically several protein(/polypeptide) O-mannosyltransferases, which often function as complexes. Part of the knock-outs are harmfull, at least for cell structures and stability and not all yeast knock-outs or combinations are tolerated (for a review, see Goto 2007, Biosci. Biotechnol. Biochem. 71(6), 1415-1427).
There have been reports of knock-outs of yeast O-mannosyltransferase genes, aiming to reduce the O-mannosylation levels, and even multiple knock-out mutants involving two or three pmt genes in S. cerevisiae (WO/1994/004687). Pmt1 or pmt2 knock-out of S. cerevisiae reduced the level of O-mannosylation of antifreeze glycoprotein III to about 30% of the proteins and the residual mannosylated protein contains numerous mannose residues per protein, apparently also oligosaccharides (WO/2004/057007).
WO/2010/034708 reports no significant level of O-mannosylation of recombinant hydrophobin Trichoderma protein when expressed in pmt1 knock-out of S. cerevisiae host cell. Such O-mannosylation appears to be artificial yeast glycosylation of the original non-mannosylated filamentous fungal protein.
WO/2010/128143 further reports single chain antibody-albumin fusion construct in yeast S. cerevisiae pmt1 and/or pmt4 knock-out strains.
Pmt1, pmt2, and pmt3 single gene knock-outs, double, and triple knock-outs of Aspergillus species (Aspergillus nidulans, Aspergillus fumigatus, and/or Aspergillus awamori) are described in Goto et al, 2009 (Eukaryotic cell 2009, 8(10):1465); Mouyna et al, 2010 (Molecular Microbiology 2010, 76(5), 1205-1221); Zhou et al, 2007 (Eukaryotic cell 2007, 6(12):2260); Oka et al, 2004 (Microbiology 2004, 150, 1973-1982); Kriangkripipat et al, 2009; Fang et al, 2010 (Glycobiology, 2010, vol. 20 pp 542-552); and Oka et al, 2005 (Microbiology 2005, 151, 3657-3667).
Despite numerous reports on knock out of pmt homologues in filamentous fungi, there is no description of a filamentous fungal cell with reduced O-mannosylation and useful as a host cell for the production of recombinant glycoprotein.
In particular, Gorka-Niec et al (2008, Acta Biochimica Polonica, Vol. 55 No 2/2008, 251-259) reported the deletion of pmt1 gene in Trichoderma reesei. PMT1 protein showed the highest identity to Pmt4p of S. cerevisiae (51%) but functionally complement pmt2Δ S. cerevisiae mutant (Gorka-Niec et al, 2007, Biochimica et Biophysica Acta 1770, 2007, 774-780). However, the authors reported that disruption of the pmt1 gene caused a decrease of protein secretion but did not alter O- and N-glycosylation of secreted protein.
Zakrzewska et al (Curr Genet 2003 43: 11-16) further reported that Trichoderma reesei pmt1 gene did not functionally complement pmt4Δ S. cerevisiae mutant.
In fact, deletions of the PMT genes in yeasts or filamentous fungi appears to either result in no phenotype at all or lethality or severely impaired vital functions of the cells, which would not be suitable for recombinant production of heterologous proteins, especially mammalian glycoproteins. For this reason, alternative methods such as the use of pmt inhibitors have been proposed as an alternative to pmt knock out strains (WO2009/143041).
Thus, a need remains for improved filamentous fungal cells, such as Trichoderma fungus cells, that can stably produce heterologous proteins with no or reduced O-mannosylation, such as immunoglobulins, preferably at high levels of expression.